Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices. CFMs include implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices, and devices that include a combination of such capabilities. The devices are typically used to treat patients using electrical therapy and to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include electrical leads or other electrodes in communication with sense amplifiers to monitor electrical heart activity within a patient, and often include sensors to monitor other internal patient parameters. Other examples of implantable medical devices include implantable insulin pumps or devices implanted to administer drugs to a patient.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these cardiac signals typically include depolarizations indicative of heart chamber contractions or repolarizations indicative of heart chamber expansions. By monitoring cardiac signals indicative of expansions or contractions, some IMDs are able to detect a tachyarrhythmia. Such IMDs typically provide therapy for tachyarrhythmia, such as high energy shock therapy or anti-tachycardia pacing (ATP). Tachyarrhythmia includes abnormally rapid heart rate, referred to as tachycardia, including ventricular tachycardia (VT) and supraventricular tachycardia (SVT), or even incoherent fibrillation.
Typically, arrhythmia detection and therapy decisions in current implantable medical devices (IMDs) are solely based on intra-cardiac electrogram (EGM) information, such as the duration of cardiac cycle intervals or morphology of detected depolarizations. A typical interval-based method compares the time interval between successive depolarizations (or, inversely, a rate-based method compares heart rate) to various zones to discriminate between or classify tachyarrhythmias. A morphology-based method typically compares the shape of a cardiac depolarization to a morphology template to discriminate between or classify tachyarrhythmias.